Reproductive and metabolic effects of recombinant equine leptin on seasonally anovulatory mares

Mares of poor body condition have low plasma leptin concentrations, while obese mares have the highest concentrations. Leptin is a primary signal of body condition to the brain in other species; therefore, low leptin concentrations in thin mares could contribute to their extended anovulatory period in winter compared to obese mares. The current experiment was designed to determine whether recombinant equine leptin, administered to seasonally anovulatory mares, would induce ovarian activity and ovulation in the winter. Leptin effects on metabolism were also studied. Beginning January 7, leptin-treated mares (n = 9) received daily i.m. injections of 10 mg recombinant equine leptin in saline and control mares (n = 10) received equivalent injections of gelatin. Reproductive effects were assessed by daily blood samples and regular ultrasound examination of the ovaries. Weights were also collected routinely. In addition, mares were confined to individual pens and hay consumption was measured as a means of evaluating appetite. A 24-h period of 30-min blood sampling was used to characterize hormone patterns. Over the course of the experiment, there was no difference between groups in follicular activity, date of first ovulation, or prolactin in either daily or frequently collected blood samples. Leptin-treated mares lost more (P \u3c 0.0001) weight than control mares in the first 31 d; however, there was no difference in appetite as indicated by 24-h hay consumption. Post-experimental analysis revealed that leptin-treated mares developed antibodies (P \u3c 0.001) against the injected leptin beginning around d 16. Subsequent leptin treatment did not affect growth hormone secretion during the frequent blood sampling window. Leptin treatment did not affect daily triiodothyronine concentrations; however, treated mares had lower (P \u3c 0.016) daily thyroxine concentrations than control mares. Treated mares also had a tendency (P = 0.11) to have lower insulin concentrations during the frequent sampling period. In conclusion, daily treatment with recombinant equine leptin had an immediate effect on body weight without any effect on hay consumption. Although leptin-treated mares had lower thyroxine concentrations and a tendency for lower insulin, no effect was observed on reproductive endpoints in the time period studied